The invention generally relates to a method of forming epitaxial layers of semiconducting materials, metals, and insulators, which may be used in the production of photodetectors and photovoltaic cells.
Photovoltaic cells have many applications. Solar cell systems may be connected to an electric utility grid or be used independently. Applications include water heating, residential electric power, electric power for buildings, generation of power for electric utilities, applications in space, military applications, electric power for automobiles, airplanes, etc., and low-power specialty applications. Solar cells may be used in rooftop systems, in sheets rolled out on large flat areas in the desert or elsewhere, on systems that track the motion of the sun to gain the maximum incident solar power, with or without lenses and/or curved reflectors to concentrate the sun's light on small cells, in folding arrays on satellites and spacecraft, on the surfaces of automobiles, aircraft and other objects and even embedded in fabric for clothing, tents, etc.
The primary function of a photovoltaic cell is to convert electromagnetic radiation, in particular solar radiation, into electrical energy. The energy delivered by solar radiation at the earth's surface primarily contains photons of energy hv in the range 0.7 eV up to 3.5 eV, mostly in the visible range, with hv related to the wavelength λ of the light by hv=1.24 eV/λ (μm). Although many photons of longer wavelength are incident at the earth's surface they carry little energy.
In a semiconductor, the lowest conduction band and the highest valence band are separated in energy by a bandgap, Eg. A semiconductor is transparent to electromagnetic radiation with photons of energy hv less than Eg. On the other hand, electromagnetic radiation with hv≧Eg is absorbed. When a photon is absorbed in a semiconductor, an electron is optically excited out of the valence band into the conduction band, leaving a hole (an absence of an electron in a state that normally is filled by an electron) behind. Optical absorption in semiconductors is characterized by the absorption coefficient. The optical process is known as electron-hole pair generation. Electron-hole pairs in semiconductors tend to recombine by releasing thermal energy (phonons) or electromagnetic radiation (photons) with the conservation of energy and momentum.
Most semiconductor devices, including semiconductor solar cells, are based on the p-n junction diode. When incident photons with energy greater than or equal to the bandgap of the semiconductor p-n junction diode are absorbed, electron-hole pairs are generated. Electron-hole pairs generated by the incident photons with energy greater than the bandgap are called hot carriers. These photo-generated hot electrons and holes, which in direct-bandgap semiconductors reside in the energy band away from the energy band zone center, rapidly give away their excess energy (the energy difference between the total carrier energy and the energy gap) to the semiconductor crystal lattice causing crystal lattice vibrations (phonons), which produce an amount of heat equal to the excess energy of the carriers in the semiconductor. As a result of the photo-generated electrons and holes moving in opposite directions under an electric field within the semiconductor p-n junction diode, electron and hole photocurrents are simultaneously generated. Semiconductor devices based on this operating principle are known as photodiodes. Semiconductor photovoltaic solar cells are based on the same operating principle as the semiconductor p-n junction photodiodes described above.
In order to achieve the highest overall efficiency, photovoltaic cells may comprise a number of subcells that are stacked on top of one another. As the light passes from the incident face of the photovoltaic cell, the light passes through the stacked subcells, each of which has a subsequently smaller energy gap. This grading of the energy gaps from cell to cell reduces the energy lost as heat and increases the overall efficiency of the photovoltaic cell.
The most common method for forming multijunction solar cell structures is to grow successive epilayers of semiconductor material as by metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) with increasing bandgaps on a substrate. Thus, the lowest cell having the lowest bandgap is grown first and subsequent subcells with higher bandgaps are grown on top of the first subcell. Usually the substrate has the lowest energy gap and is used as the bottom subcell in a solar cell.
Alternatively, U.S. Pat. No. 6,951,819 to Iles et al. describes a method of forming a Group III-V solar cell wherein the first epilayer grown has the highest bandgap and each subsequent epilayer grown has a smaller bandgap than the epilayer below it. However, column 1, lines 37-63 of Iles states that the possible bandgap values are limited because the crystal structure and lattice constants of the different layers of the Group III-V materials must be matched to each other in order to maintain the necessary electronic properties. Stated another way, each layer of Group III-V material must have essentially the same lattice constant, or distance between neighboring atoms, as every other layer and as the substrate in order to obtain a high-efficiency solar cell. The lattice constant of each layer, however, is affected by the chemical composition of the layer because different sizes of atoms give different interatomic distances.
Thus, the requirement of matching the lattice constant of each layer to that of the substrate directly limits the allowed compositions of the layers and, therefore, the possible bandgaps. If the lattices did not have to be matched, different compositions could be used to adjust the bandgap of each layer. This would lead to more efficient photovoltaic cells.
Thus, there is a need for a process for forming semiconducting layers that allows one to choose the bandgap for each layer without regard to lattice matching, while maintaining acceptable electronic properties.